1. Field of the Invention
The invention relates to an electric power conversion system and, more particularly, to an electric power conversion system that includes a primary electric power conversion circuit and a secondary electric power conversion circuit magnetically coupled to the primary electric power conversion circuit.
2. Description of Related Art
There has been developed an electric power conversion system that bidirectionally exchanges electric power between a primary electric power conversion circuit and a secondary electric power conversion circuit magnetically coupled to the primary electric power conversion circuit. Each of the primary electric power conversion circuit and the secondary electric power conversion circuit is configured to include a plurality of transistors that are serially connected to each other between a positive electrode bus and a negative electrode bus. In each of the primary electric power conversion circuit and the secondary electric power conversion circuit, a dead time during which both the serially connected transistors are turned off is provided in order to prevent a short circuit between the plurality of serially connected transistors.
As a related technique of the invention, for example, U.S. Pat. No. 7,408,794 describes an electric power conversion circuit that includes three input/output ports and half-bridge circuits. There is also disclosed that a high-voltage inverter circuit, a 14V load and a 42V load are respectively connected to the three input/output ports of the electric power conversion circuit.
Japanese Patent Application Publication No. 2006-187147 (JP 2006-187147 A) describes an electric power conversion circuit that includes two input/output ports and full-bridge circuits. The electric power conversion circuit includes three reactors in addition to reactors that constitute a transformer. There is also disclosed that a high-voltage inverter circuit and a low-voltage electronic device are respectively connected to the two input/output ports of the electric power conversion circuit.
In the above electric power conversion system, a power transfer direction and a power transfer amount are controlled by equalizing the terminal voltage waveform of a primary coil of the primary electric power conversion circuit to the terminal voltage waveform of a secondary coil of the secondary electric power conversion circuit and then adjusting a phase difference φ between the two voltage waveforms.
When a dead time is provided as described above, the phase symmetry between the terminal voltage waveform of the primary coil of the primary electric power conversion circuit and the terminal voltage waveform of the secondary coil of the secondary electric power conversion circuit may fluctuate.
Due to fluctuations in voltage of a power supply connected to the primary electric power conversion circuit or secondary electric power conversion circuit, the voltage ratio between the terminal voltage waveform of the primary coil and the terminal voltage waveform of the secondary coil may fluctuate. In this way, when there occur the above-described phase symmetry fluctuations or voltage ratio fluctuations, circulating current that does not contribute to transfer of electric power flows in the primary electric power conversion circuit and the secondary electric power conversion circuit.